For the preparation of coffee, in particular of espresso coffee, that is to say of an infusion obtained from hot water and coffee powder, apparatuses of various classes are used, which in every case provide for causing water at high temperature, raised to a sufficient pressure, to pass through a layer of coffee powder, then collecting the infusion thus obtained.
To produce a coffee of high quality, it is desirable that the infusion water shall be at a temperature lower than boiling temperature, in order to avoid removing from the coffee powder, in addition to the aromatic components which give flavor to the beverage, also undesirable components and even at times harmful components, which only leave the coffee powder at temperatures greater than the boiling point of water.
Considerable use is therefore made, both in public buildings and in private dwellings, of apparatuses which, by feeding water through the powder without bringing it to boiling point, enable drinks of high quality to be obtained.
In particular, for domestic use, a machine is required for the preparation of coffee which shall be compact and of limited cost.
The automatic machines for producing coffee provide for locating the coffee powder inside a filter-carrying element, equipped with a filter for retaining the coffee powder and allowing the infusion produced to pass through, for pressing the powder with a piston to a suitable degree and for feeding, by means of said piston, pressurized hot water for producing the infusion.
For this purpose, there should be provided a motor device which shall cause insertion of the piston into the filter-carrying element and maintain its position during the feeding of pressurized hot water.
For this purpose, use may be made of actuating units with geared motors of an irreversible type, but such geared motors are costly and are not well suited for a machine for domestic use.
During the automatic operation of machines of the type described, the problem furthermore arises of proper cleaning of the filter and of the filter-carrying element to remove exhausted powder, so that new powder may be introduced into it.
In the case of preparation of coffee, such exhausted powder is usually present in the form of a pressed, compact cake, which can be easily thrust laterally without leaving traces; in some cases, however, for example in particular conditions of humidity or the like, the cake of exhausted powder can crumble, leaving a part of the powder on the filter-carrying element; furthermore, when the machine is used for the production of infusions other than of coffee, such as tea or the like, residues such as tea leaves, which are humid and adhere to surfaces may be left.
In these conditions, therefore, it may happen that residues persist in the machine, which, if not completely removed, would alter the taste of the next beverage.